Creating photolithographic masks

ABSTRACT

An embodiment of the present invention described and shown in the specification is a system for optimizing data used in creating a photolithographic mask. The system reads a definition of a layer of wafer to be created with a photolithographic mask and defines a number of polygons corresponding to conventional patterns on a mask and polygons corresponding to areas on the mask that are phase shifters. A number of data layers are created and the polygons that define phase shifting areas that shift the phase of light by differing amounts of are grouped in different data layers. Once separated, the system analyzes the polygons in each data layer against one or more design rules and assigns a phase shift amount to all the polygons in a data layer in accordance with the analysis. The polygon definitions in each data layer are then given to a mask maker to fabricate a photolithographic mask. It is emphasized that this abstract is being provided to comply with the rules requiring an abstract and will not be used to interpret or limit the scope or meaning of the claims under 37 C.F.R. § 1.72(b).

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 09/703,294,filed Oct. 31, 2000 now U.S. Pat. No. 6,728,946, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to photolithographic processing.

BACKGROUND OF THE INVENTION

In order to create faster and more powerful integrated circuits, circuitdesigners are increasing the number and decreasing the size of circuitelements that are placed on an integrated circuit. With conventionalphotolithography, the minimum size of an object that can be created on asilicon wafer depends directly on the wavelength of light used to exposethe wafer and inversely on the numerical aperture of the lens throughwhich the light that exposes the wafer is passed. Because the costsassociated with decreasing the illumination wavelength or increasing thenumerical aperture can be prohibitive, chip manufacturers arecontinually looking for techniques that can create smaller objects on awafer using existing photolithographic equipment.

One of the most powerful techniques for increasing the density of anintegrated circuit with existing photolithographic equipment is with theuse of phase shifters. As discovered by Marc Levenson of IBM and others,the phase of the light that strikes a wafer can be manipulated todestructively interfere at desired locations on the wafer in order toenhance image contrast and reduce diffraction effects that occur whenthe light passes through a pattern of opaque areas on a semiconductormask. In addition, by selectively placing phase shifters on the mask,subwavelength features can be created on the wafer to form circuitelements.

While the use of phase shifting structures on a photolithographic maskallows increased contrast and the creation of subwavelength featuresusing existing photolithographic equipment, the phase shifters arerelatively expensive to create and may introduce errors into the mask.Therefore, there is a need for a method and apparatus for optimizing thecreation of phase shifters on a mask that minimizes the possibility oferrors and facilitates the production of the mask.

SUMMARY OF THE INVENTION

A method and apparatus for optimizing data used in the creation of aphotolithographic mask. The apparatus includes a computer system forreading data that describes a desired physical layer of an integratedcircuit. The computer creates multiple data layers and groups datastructures that define different areas on the mask into one of themultiple data layers. After the data structures are defined and groupedinto one of the data layers, the data structures grouped in one or moreof the data layers are analyzed according to one or more design rules.One or more properties of the data structures are then assigned inaccordance with the analysis performed.

In a currently preferred embodiment of the invention, the invention isused to optimize data that defines phase shifters on a photolithographicmask. The computer system creates data structures that define polygonsthat correspond to circuit elements and polygons that correspond tophase shifters on the mask. Polygons are then placed in one of thecorresponding data layers. All the polygons associated with circuitfeatures are grouped in one data layer and the polygons associated withdistinct phase shift areas are grouped in separate data layers. Thephase shifting polygons in the data layers are analyzed and a phaseshift amount is assigned to each of the polygons after the polygons havebeen created, and based on the design rule analysis. In one embodimentof the invention, the design rules operate to minimize an etched area ofthe mask.

The present invention is not limited to the optimization of data used tocreate masks for integrated circuits. The present invention can also beused to optimize data used to create micro electro-mechanical structures(MEMS), thin film heads for disk drives or other devices created withphotolithographic processes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIGS. 1A–1H illustrate cross sections of various types of known phaseshifting elements that may be incorporated into a photolithographicmask;

FIGS. 2A and 2B illustrate a portion of a desired circuit and how phaseshift values are conventionally assigned to adjacent phase shiftingareas on a mask;

FIG. 3 is a flow chart of the conventional steps performed to assignphase shift values to areas of a phase shifter;

FIGS. 4A–4B illustrate a flow chart of the steps performed by oneembodiment of the present invention to optimize the assignment of phaseshift values to areas of phase shifters;

FIG. 5 illustrates a computer system for performing the method outlinedin FIG. 4;

FIGS. 6A–6G illustrate a portion of a desired circuit and how thepresent invention optimizes the assignment of phase shift values; and

FIG. 7 illustrates the use of the present invention with attenuatedphase shifters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As indicated above, the present invention is a method and apparatus foroptimizing data for the creation of circuit elements on aphotolithographic mask and, in particular to the optimization of datafor the creation of phase shifters on a photolithographic mask.

FIGS. 1A–1H illustrate various types of structures commonly used onphotolithographic masks. As shown in FIG. 1A, a mask 10 typicallycomprises a transparent substrate, typically quartz or glass, having anumber of opaque areas 12, typically fabricated from chrome, disposed onone surface thereof. This is typically called a conventional,chrome-on-glass (COG) mask. The mask 10 is placed into a waferprocessing machine whereby light is passed through the mask 10 and animage of the mask is formed on the surface of a silicon wafer (notshown) that is covered with a photosensitive material. The light exposesportions of the photosensitive material and the exposed/unexposed areasof the photoresist material are then processed in order to create apattern on the wafer corresponding to the pattern of opaque areas 12 onthe corresponding mask 10.

In order to improve the contrast between adjacent features on the waferor to create features that are smaller than the wavelength of the lightused to expose the wafer, the mask 10 may include one or more phaseshifters, as shown in FIG. 1B. Light passing directly through atransparent area of the mask 10 as indicated by the beam 14 forms areference against which the phase of phase shifted illumination light iscompared. Clear areas of the mask are typically referred to as zerodegree (0°) areas. The phase of the light passing through the mask 10may be adjusted by adding a layer of transparent material 16 to thesurface of the mask 10. The thickness of the material 16 added can bevaried in order to control the phase of the light passing through thatarea of the mask 10 with respect to the phase of the light passingthrough a 0° area of the mask 10. For example, light passing through anadditional layer of material 16, as indicated by the beam 18, is 180°out of phase with the light beam 14. When light beams that are 180° outof phase intersect on the silicon wafer, the lightwaves destructivelyinterfere, thereby enhancing the contrast between exposed portions onthe wafers that are created by adjacent opaque areas 12 on the mask 10.

The phase of the light passing through the mask also can be adjusted byetching an area into the mask in order to reduce its thickness. FIG. 1Cshows a mask 10 having an area 20 that has been etched to shift by 180°the phase of the light passing through the area as indicated by thelight beam 22.

As long as the relative phase shift between adjacent phase shiftingareas is 180°, the contrast enhancement is still achieved. FIG. 1D showsa mask 10 having an area 24 that has been etched to shift the phase ofthe illumination light by 90° with respect to light that passes througha 0° area of the mask. The mask 10 also includes an area 26 that isetched to shift the phase of the illumination light by 270°. Lightpassing through a 0° area, indicated by the light beam 14, and a 90°area, as indicated by the light beam 28, will not destructivelyinterfere to create a sub-wavelength feature on the wafer. Similarly,light passing through the 270° phase shift area, as indicated by thelight beam 30, will not destructively interfere with light passingthrough a 0° portion of the mask 10 to create sub-wavelength features.Only light passing through adjacent 90° and 270° phase shift areas willcreate sub-wavelength features. Using 90° and 270° phase shifters allowsgreater flexibility in the layout of phase shifting masks. However,having phase shifters etched to different depths on the mask generallymakes the mask more expensive to manufacture.

The use of phase shifters is not limited to enhancing the contrastbetween adjacent apertures separated by opaque areas on a mask. A phaseshifting area can be placed inside a clear area on the mask in order tocreate sub-wavelength features on the wafer. FIG. 1E illustrates a mask10 having a 0° area 34 that is adjacent a 180° phase shift area 36.Light passing through these areas, as indicated by the beams 38 and 40,destructively interfere at the wafer thereby producing an unexposedpattern that can form a desired element on the wafer.

As is well known to those of ordinary skill in the art, at each locationon the mask where a 180° phase shift area borders a 0° phase shift area,destructive interference will create a sub-wavelength feature on thewafer. Because each phase shifting region must have a closed boundary,additional features may appear in undesirable locations. These undesiredfeatures can be removed with a trim mask 11 having opaque areas 39, 41,as shown in FIG. 1F that shield desired portions of the sub-wavelengthfeatures and expose undesired portions of the sub-wavelength features.

FIG. 1G illustrates the use of multiple adjacent phase shifting areas ona mask 10. The mask 10 includes a 0° phase shifting area 42 that isadjacent to a 180° phase shifting area 44 that, in turn, is adjacent asequence of 90° phase shifting areas 46. Light passing through the 0°and 180° degree areas, as indicated by beams 48 and 50, willdestructively interfere on the wafer to create a sub-wavelengthunexposed area. Light passing through the 180°0 phase shift areaindicated by the beam 50 and the 90° phase shift area as indicated bythe beam 52 or at the boundary of the 90° and 0° regions will notinterfere to create a sub-wavelength feature. Thus, placing a 90° phaseshift area between a 0° and a 180° phase shift area on the mask 10 canreduce or eliminate the unwanted sub-wavelength features that wouldotherwise be created at the boundaries of a 180° and 0° phase shift areawithout a trim mask. However, these masks are more complex and thereforemore expensive.

An alternate approach of creating a phase shifter is to alter thematerial used to make the opaque patterns on the mask 10. Such patternsare generally created with opaque chromium. By making the chromium thinenough, or using specially developed partially transmitting materialssuch as Molybdenum silicide, the material becomes semi-transparent andalters the phase of the light passing through it by 180°, but decreasesits amplitude. FIG. 1H shows a mask 10 having a 0° phase shift area 56and a semi-transparent pattern 58. Light passing through thesemi-transparent pattern 58 is shifted 180° with respect to lightpassing through the 0° area 56 of the mask in order to enhance thecontrast between adjacent features. Uniform phase shifting can beachieved by creating a uniform layer of the material.

As will be appreciated, the semi-transparent pattern 58 increases theoverall amount of light that reaches the wafer. Therefore, portions ofthe semi-transparent pattern may be covered with an opaque chrome layerto produce a “tri-tone” mask as will be described in further detailbelow.

While the use of phase shifters permits integrated circuits with anincreased density to be created with existing photolithographicequipment, their use generally increases the cost of producing the maskand may introduce errors into the mask. FIGS. 2A–2B and 3 illustrate theconventional method by which phase shifters are added to aphotolithographic mask. FIG. 2A shows a portion of a desired circuit tobe created on a silicon wafer. The circuit includes a number ofpolysilicon elements 80 a–80 e that are large enough to be created by aconventional COG photolithographic mask. In addition, the circuitincludes two gates 82 a and 82 b that are sub-wavelength features.

To prepare the data for a mask that will create the desired layout on asilicon wafer, the process steps shown in FIG. 3 are usually performed.It should be noted that the term “mask” in photolithographic processingtechnically refers to an object that is placed in direct contact with awafer during processing, as opposed to a reticle which is positioned atsome distance away from the wafer during processing. However, forpurposes of the present specification, the term “mask” is intended tohave its more colloquial definition and refer to both contact masks andreticles. Beginning with a step 100, a computer file containing adescription of the circuit layout is received and provided to a layoutmanipulation program. A layout manipulation and verification programtakes the description of the circuit elements and defines a number ofpolygons that in turn correspond to areas on one or morephotolithographic masks at a step 102. For example, to create thecircuit element 80 a, the layout manipulation program defines a polygonp80 a (FIG. 2B) that describes the dimensions of a portion of a maskthat will create the element 80 a when the mask is illuminated. Inaddition, the layout manipulation program detects that the gates 82 aand 82 b are too small to be created using a convention mask andtherefore generates polygons that ensure that the mask includes phaseshifting regions 84, 86 and 88 that will create these sub-wavelengthfeatures by destructive interference. The description of the polygonsthat define areas the mask is generally referred to as a GDS II datalayer for the format of a language commonly used to describe thepolygons, although other data formats can also be used.

At a step 104, the layout manipulation program makes an initial phaseselection for one of the phase shift areas. For example, if the circuitdesigner selects a design using 0° and 180° phase shift areas, thelayout manipulation program will select either 0° or 180° as the phaseshift for the first area. Processing then continues at a step 106 wherethe layout manipulation program continues through the data layerassigning each phase shift area a phase shift amount that is theopposite of its neighboring phase shift area. In the example shown inFIG. 2, the phase shift area 86 is assigned 180° of phase shift and thearea 88 is assigned 0° of phase shift.

The problem with the approach shown in FIGS. 2B and 3 is that thearbitrary phase assignments that can occur when simple rules areexecuted for the assignment of phase shift values at the same time thepolygons are generated can result in sub-optimal masks. For example,masks with large etched areas may contribute to errors on the mask. Asshown in FIG. 2B, the area 86 that is etched to produce 180° of phaseshift completely contains the polygon p80 e. Having an etched area thatcompletely surrounds a chrome feature on the mask can cause etching ofthe chrome itself, changing the dimensions and increasing the number ofdefects on the mask. Repair of these defects may be difficult and timeconsuming, increasing the cost of the mask.

To improve the creation of phase shift areas on a mask, one embodimentof the invention performs the steps shown in FIGS. 4A–4B. Beginning witha step 120, the data layer for a selected physical chip layer isobtained. For example, the physical chip layer may be the polysilicondevice layer that specifies a particular pattern of transistors to becreated on the wafer. At a step 122, a layout manipulation program, suchas is part of Calibre™ produced by Mentor Graphics Corporation ofWilsonville, Oreg., the assignee of the present invention, is executedby a computer system to determine the appropriate phase shiftingmethod—either by prompting a user to make a selection or based on one ormore design rules. At a step 124, the computer creates a number of datalayers depending on the type of phase shifters to be used. For example,if a 0° and 180° phase shifting scheme with double exposure is used, thecomputer system creates four data layers. One data layer is created forthe circuit elements that can be created with conventional patternsformed on a mask, one data layer is created for each phase shiftingportion of the phase shifters, as will be explained below, and one datalayer is created for a trim mask. If a 90°/270° phase shifting scheme isused, the computer system creates three data layers since no trim maskis needed. It should be noted that the data layers do not correspond todistinct layers of the circuit to be fabricated as distinct masks, butcorrespond to separate data structures such as arrays, files or otherstorage mechanisms in which the polygons grouped therein can beanalyzed. At the time of creation, these data layers are generallyempty, and do not yet contain any polygons.

At a step 126, the layout manipulation program creates those polygonsthat define different phase shifting areas of the phase shifters andplaces them into one of the different data layers created.

Selecting an actual phase shift amount for each of the polygons in thedifferent data layers does not yet occur. Preferably, after the polygonsare created, the assignments made. At a step 128, the layoutmanipulation program makes a phase shift selection for all the phaseshift polygons in the same data layer in accordance with a desireddesign criteria. For example, it may be advantageous to minimize thearea of the 180° phase shift areas on the mask. Therefore, the layoutmanipulation program sums the area of each phase shifting polygonincluded in each data layers. The polygons in the data layer having theminimum combined area can be defined as the 180° phase shift regions.Alternatively, the selection of which polygons define the 180° phaseshift areas may be made based on other design criteria, such asproximity to circuit elements in other layers, or the predictive resultsof a simulation algorithm.

At a step 130, the layout manipulation program may run a simulation ofthe circuit based on the phase assignments made at step 128. At a step132, it is determined if the simulation is acceptable. If not,processing can proceed to a step 134 and selected phase shiftingpolygons can be reassigned to another data layer or the phase shiftassignment for the entire data layer can be changed. After step 134,processing returns to step 130 and the simulation is performed again.Once the simulation is acceptable, processing proceeds to a step 136,whereby the polygon definitions for each of the data layers is providedas input to a mask writer to create the mask for the physical chiplayer.

FIG. 5 illustrates the basic components of a hardware system thatimplements the present invention. A database 150 stores GDS II layerdata for each physical layer of an integrated circuit to be created. Thedata is read by a computer system 152 that executes a program thatimplements the functions outlined in FIGS. 4A–4B and described above.Once the data layers have been created, the polygon definitions havebeen divided among the data layers and the phase assignments for thephase shifting areas selected and the design verified, the polygondefinitions in each data layer are supplied as input to a mask dataprocessor 154 that controls the production of the actualphotolithographic mask.

FIGS. 6A–6G illustrate how one embodiment of the present inventionoperates with an actual circuit design. FIG. 6A illustrates a portion ofa desired integrated circuit design including a number of circuitelements 200 a–200 e that are large enough to be created withconventional mask patterns. In addition, the desired circuit designincludes a gate 202 a that connects element 200 a with element 200 c anda gate 202 b, which connects circuit element 200 b with circuit element200 d. In the example shown in FIG. 6A, gates 202 a, 202 b are too smallto be created with a conventional mask. Therefore, the layoutmanipulation program knows that these elements must be created with theuse of phase shifters.

FIG. 6B illustrates polygons created by the layout manipulation programin order to produce the mask or masks that will in turn be used tocreate the circuit elements shown in FIG. 6A. The layout manipulationprogram defines a series of polygons p200 a–p200 e that correspond tothe circuit elements 200 a–200 e, respectively. Each of these polygons,for example polygon p200 a, defines an area on the mask that will createthe corresponding element 200 a on the wafer without the use of thephase shifter. If the mask is a bright field mask, the polygon p200 awill be defined as an opaque area on the mask surrounded by clear or 0°phase shift areas. If the mask is a dark field mask, the area of thepolygon p200 a will be defined as clear and surrounded by opaque areas.In addition, the layout manipulation program defines polygons p204, p206and p208 that correspond to phase shift areas on the mask that togetherwill create the two sub-wavelength gates 202 a and 202 b. The sizes ofthe polygons p204, p206, and p208 are minimized as compared with thesize of the phase shifting regions 84, 86 and 88 created withconventional techniques as shown in FIG. 2.

As indicated above, one embodiment of the present invention separatesthe polygons created into one of several data layers. Each polygon thatcorresponds to a conventional pattern on the mask is placed in one datalayer as shown in FIG. 6C. In addition, the polygons that definedifferent phase shifting areas of a phase shifter are placed indifferent data layers. For example, polygons p204 and p208 are placed inone data layer as shown in FIG. 6D and the polygon p206 for the otherphase shifting area is placed in another data layer as shown in FIG. 6E.

Once the polygons associated with different phase shift areas are placedin separate data layers, the layout manipulation program makes a phaseselection for the polygons in the data layers according to a desireddesign criteria. For example, if the design criteria specifies that thearea of etched phase shift regions is to be minimized, the computersystem sums the area of the polygons p204 and p208 within the data layershown in FIG. 6D and compares the total area with the summed area of thepolygons contained in the data layer shown in FIG. 6E. Depending uponwhich data layer has the polygons with the smallest summed area, thosepolygons can be selected to have a 180° etched phase shift. Once thecomputer system has made an initial phase shift selection for all thepolygons in a data layer, a simulation can be performed on the circuitlayout to ensure that the circuit will perform as desired.

As will be appreciated, depending upon the selection of the phase shiftvalues for the various polygons that define the phase shifters, theappropriate trim masks can then be defined. For example, if the polygonp204 and p208 are selected to have 180° phase shifts, then subwavelengthartifacts will be created in the areas 210 and 212 is shown by thedashed lines in FIG. 6B. Therefore, a data layer that defines a trimmask as shown in FIG. 6F can be created by the layout manipulationprogram. The trim mask is opaque except for the areas 220 and 222.Illumination of the wafer through the trim mask removes the undesiredartifacts created at the boundaries of a 0° and 180° phase shift area.Alternatively, if the polygon p206 is selected to have a 180° phaseshift, then artifacts will be created in the areas 214 and 216 shown bythe dashed lines in FIG. 6B. Therefore, a data layer that defines a trimmask as shown in FIG. 6G is created. The trim mask is opaque except forareas 224 and 226.

As will be appreciated, trim masks are only required if 0° and 180°phase shifters are used. If alternate phase shifting schemes such as90°/270° or 60°/120° phase shifters are used, then no trim masks need becreated.

The present invention is not limited to optimizing phase shifters thatlie side by side on the mask. As indicated above, phase shifting may beaccomplished by using a attenuating phase shifting material on the maskto define circuit elements. Typically, these masks are made using amolybdenum silicide material, in which the thickness and opticalproperties are controlled to allow the transmission to be small,typically 6–9%, and phase shifted by 180°. In order to limit the totalamount of light that passes through the mask, portions of theattenuating material can be covered with an opaque chrome to form atri-tone mask. FIG. 7 illustrates a plan view of a portion of a maskwherein the mask contains a pattern of attenuating material, which issometimes referred to as “leaky chrome”. In addition some areas of theattenuating material may be covered with an opaque chrome 252. Thepresent invention can optimize the placement of the opaque chrome overthe leaky chrome by creating separate data layers for the polygons thatdefine the leaky chrome areas and for the polygons that define the areasof opaque chrome. With the polygons for each area separated, the layoutmanipulation program can perform calculations to optimize the size ofthe opaque areas, while still getting the benefits of the phase shiftingareas due to the light passing through the attenuating material. Oncethe polygons in each of the data layers have been optimized, the datalayers are provided as input to the mask writer to create the differentlayers on the photolithographic mask.

While the preferred embodiment of the invention has been illustrated anddescribed, it will be appreciated that various changes can be madetherein without departing from the scope of the invention. For example,although use of the present invention is illustrated with respect tocreating integrated circuits, it will be appreciated that the presentinvention can also be used to create micro electro-mechanical structures(MEMS), thin film disk drive heads or other structures created usingphotolithographic techniques. Similarly, data structures that definedifferent areas of a phase shifter need not be grouped in different datalayers. The data structures could be grouped in a common data layer andtagged with an identifier that allows the computer to identify it andperform an optimization calculation prior to assigning a phase shiftvalue. Once the phase shift values are assigned, the different polygonsfor the phase shift areas having the same phase shift value arepreferably written to a single data layer and provided to a maskprocessor such that the same process steps can be performed on eachpolygon in the data layer. It is therefore intended that the scope ofthe invention be determined from the following claims and equivalentsthereto.

1. A method for optimizing data to create one or more photolithographicmasks using a layout database having a number of data layers,comprising: receiving data that represents features to be created in aphysical layer of an integrated circuit; creating a number of datastructures that represent regions of a mask and assigning each datastructure to a data layer in a layout database such that data structuresare grouped in two or more layers of the layout database; analyzing thedata structures that are grouped within to the same data layer accordingto one or more design rules after the data structures have been created;and fixing a property of each data structure that is grouped within thesame data layer in accordance with the analysis performed such that eachdata structure in the same data layer has the same property.
 2. Themethod of claim 1, wherein at least some of the data structuresrepresent phase shifting areas on a mask, wherein the data structuresthat represent adjacent phase shifting areas on the mask are assigned todifferent data layers in the layout database.
 3. The method of claim 2,wherein the property that is fixed for each data structure thatrepresents a phase shifting area is a phase shift amount, and whereinall data structures that represent phase shifting areas within the samedata layer are assigned the same phase shift amount.
 4. The method ofclaim 3, wherein the phase shift amount requires that the mask be etchedand the design rules select the phase shift amount for the phaseshifting areas to minimize the area etched on the mask.
 5. The method ofclaim 3, wherein the phase shift amount requires the application ofadditional transparent material on the mask, and the design rules selectthe phase shift amount for the phase shifting areas to minimize theamount of additional transparent material on the mask.
 6. The method ofclaim 3, wherein the property that is fixed for each data structure thatrepresents a phase-shifting area is 180 degrees.
 7. The method of claim3, wherein the property that is fixed for each data structure thatrepresents a phase-shifting area is 270 degrees.
 8. The method of claim3, wherein the property that is fixed for each data structure thatrepresents a phase-shifting area is 90 degrees.
 9. The method of claim2, wherein the property that is fixed for each data structure thatrepresents a phase-shifting area is an amount by which the phaseshifting region attenuates transmitted light.
 10. The method of claim 1,wherein at least some of the data structures define areas on the maskthat are covered by a partially transparent material and are assigned toa first data layer, and some of the data structures define areas on themask that overlay an area of a partially transparent material with anopaque material and are assigned to a second data layer that isdifferent from the first data layer.
 11. The method of claim 1, furthercomprising the step of: performing a lithographic simulationcorresponding to the data structures with the properties assigned. 12.The method of claim 11, further comprising the step of detecting errorsin the lithographic simulation and reassigning one or more datastructures to another data layer and re-analyzing the data structures inthe same data layer according to one or more design rules and refixingthe properties of the data structures in the data layer in an iterativeprocess to eliminate any errors.
 13. The method of claim 1, wherein thedata structures are polygons.
 14. The method of claim 1, wherein thephysical layer is a gate layer.
 15. The method of claim 1, wherein thephysical layer is an interconnect layer.
 16. A method of optimizing datathat define phase shifting areas on a photolithographic mask,comprising: receiving data that describes features of a physical layerto be created on an integrated circuit; creating from the data a numberof: data structures that represent areas on the mask that will be opaqueor non-opaque to create circuit elements; and data structures thatrepresent phase shifting regions on the masks, each data structure thatrepresents a phase shifting region having a phase shift amount property;assigning the data structures to data layers in a layout database, suchthat data structures that represent adjacent phase shifting regions aregrouped in two or more different data layers of layout database;analyzing the data structures assigned to the same data layer inaccordance with one or more design rules after the data structures havebeen created; and assigning a common phase shift amount property for allthe data structures that represent phase shifting regions and areassigned to the same data layer in accordance with the analysisperformed.
 17. The method of claim 16, wherein the phase shift amountproperty of the data structure represents a degree of etching on themask, and wherein the one or more design rules minimize the etched areaon the mask.
 18. A method for creating data used to produce one or morephotolithographic masks, comprising: receiving data that representsfeatures in a layer of a wafer to be created with the one or morephotolithographic masks; creating a number of data structures thatrepresent phase shifting areas on the one or more photolithographicmasks; dividing the data structures that represent phase shifting areasinto groups such that data structures that represent adjacent phaseshifting regions are divided into different groups assigning the datastructures that are divided into different groups to different datalayers of a layout database; analyzing the data structures that arecommonly grouped with one or more design rules after the data structureshave been created; and assigning a common property to each datastructure that is commonly grouped in the same layer of the layoutdatabase in accordance with the analysis performed.
 19. The method ofclaim 18, wherein the analysis is performed on the data structuresgrouped within a same data layer and wherein each data structure in thesame data layer is assigned the common property.
 20. The method of claim19, wherein the property is a phase shift amount.
 21. The method ofclaim 20, wherein the phase shift amount is 180 degrees.
 22. The methodof claim 20, wherein the phase shift amount is 270 degrees.
 23. Themethod of claim 20, wherein the phase shift amount is 90 degrees. 24.The method of claim 19, wherein the property is an amount by which aphase shifting area attenuates transmitted light.
 25. A system forcreating data used to produce one or more photolithographic masks,comprising: a database on which is stored data that defines a number oflayers of a wafer to be created with the one or more photolithographicmasks; a computer system that executes a sequence of programmedinstructions to perform the acts of: reading data from the database thatdefines a number of features to be created in a layer of the wafer withthe one or more photolithographic masks; creating a number of datastructures that represent phase shifting areas on the one or morephotolithographic masks; grouping the data structures that representphase shifting areas such that the data structures for adjacent phaseshifting areas can be analyzed separately; analyzing the commonlygrouped data structures with one or more design rules after the datastructures have been created; and assigning the same property to eachcommonly grouped data structure in accordance with the analysisperformed.
 26. A computer readable media on which is stored a sequenceof programmed instructions that when executed by a computer, cause it toperform the acts of: receiving data that represents features in a layerof a wafer to be created with the one or more photolithographic masks;creating a number of data structures that represent phase shifting areason the one or more photolithographic masks; dividing the data structuresthat represent phase shifting areas into groups such that datastructures that represent adjacent phase shifting regions are dividedinto different groups; analyzing the data structures that are commonlygrouped with one or more design rules after the data structures havebeen created; and assigning a property of each data structure that iscommonly grouped in accordance with the analysis performed such thateach of the commonly grouped data structures is assigned the sameproperty.
 27. A system for producing one or more photolithographicmasks, comprising: means for storing data that defines one or morelayers of a wafer to be created with the one or more photolithographicmasks; computer means for receiving the data and creating a number ofdata structures that represent areas on the one or morephotolithographic masks at least some of which represent phase shiftingareas, the computer means further dividing the data structures thatrepresent phase shifting areas into groups, analyzing the datastructures that are commonly grouped according to one or more designrules after the data structures are created and assigning the same phaseshift amount to the commonly grouped data structures in accordance withthe analysis performed.
 28. A photolithographic mask that is producedby: receiving data that represents features in a layer of a wafer to becreated with the one or more photolithographic masks; creating a numberof data structures that represent phase shifting areas on the one ormore photolithographic masks; dividing the data structures into groupssuch that data representing adjacent phase shifting regions are indifferent groups; analyzing the commonly grouped data structures withone or more design rules after the data structures have been created;and assigning the same property to each of the commonly grouped datastructures in accordance with the analysis performed.
 29. A computerreadable media on which a sequence of instructions are stored that areexecutable by a computer to perform any of the method claims 1–15 and6–9.
 30. A computer readable media on which a sequence of instructionsare stored that are executable by a computer to perform any of themethod claims 16–17.
 31. A computer readable media on which a sequenceof instructions are stored that are executable by a computer to performany of the method claims 18–20.